Abaddon is the Lord of Locusts and God of Death, Destruction and Nihilism. In spite of his alignment, he is worshipped by millions of beings across the world, as it is solely by his will that the spirits of the dead reach their respective planes.

Very little is known of the origins of the deity known as Abaddon, although rumours abound. Some say that he was an extremely powerful fiend who ascended to divinity prior to the start of the Blood War, while others believe that he is a manifestation of avarice and gluttony crafted by the Creator to punish the sins of mortals. In any case, one thing is clear: Abaddon is the ultimate Nihilist. He believes that all things in the multiverse exist to feed his literally all-consuming hunger. To that end, he created the Locust Swarm, an army of mindless drones commanded by his clergy, with the ability to consume any material and feed it directly to Abaddon, no matter the distance or barriers. Abaddon dwells in the Nihil Spire, a massive structure made from the molted exoskeletons of countless Ieiunoloths (Yugoloths created by him as servants) which serves as both a fortress and a prison. The Spire floats between the second and third layers of The Bleak Eternity of Gehenna.

Abaddon's temples are likely the only overt temples dedicated to malevolent deities that one would find in every major city on the world. The fittest of the clergy are forced to undergo horrendous hours of torture and mutilation disigned to erase all aspects of their personalities, aside from their loyalty to Abaddon, and to increase their tolerance for pain to almost superhuman levels. These clerics are known as Locusts, with the greatest among them being Swarm Lords.

Abaddon is a Yugoloth that underwent apotheosis and became a God. To be specific, he is a former Baernaloth, one of the progenitors of the Yugoloths, is the chief instigator of the conflict known as the Blood War, and maintains ties, however distant, with the rest of his former brethren. He has made dealings with some of the more mercenary Yugoloths to keep the war going for as long as possible, and has made several subtle attempts to get Asmodeus and Demogorgon at each other's throats; as long as the fiends are trying to slaughter each other, they aren't paying attention to what he's doing, which works to Abaddon's agenda.

Abaddon's true form is that of a gargantuan humanoid locust with the skull of a pit fiend for a head, and a single crimson eye in the center of his torso. He has four arms, which give him multiple attacks per turn, even when wielding his gargantuan Unholy Scythe.

Abaddon once aspired to become an overdeity and failed, not because he lacks the power, but because the other gods came together and threw him down, sealing him within his Spire and dashing any possibility of fulfilling Abaddon's ambition. Although he is still able to perform his duty as the Lord of Death, Abaddon is unable to meddle directly in the affairs of the Material Plane. Unfortunately, he has created two avatars to fulfill that very purpose: Seth, a medium bald humanoid, and Decimation, a Colossal centipede-scorpion hybrid.